Process of producing pure aluminium combinations adapted for the production of aluminium



Patented June 20, 1933 UNITED STATES PATENT OFFICE MAX BUCI-INER, 0FHANOVER-KL-EEFELD, GERMANY PROCESS OF PRODUCING PURE ALUMINIUMGOMBINATIONS ADAPTED FOR THE PRODUCTION OF ALUMINIUM No Drawing.Application filed February 26, 1929,

The production of aluminium metal is usually carried out in an electricfurnace by decomposition of aluminium oxide, this being brought into amelting bath consisting mainly of cryolite. Although aluminiumcombinations are found abundantly in nature, only comparatively fewdeposits supply suitable initial substances for the aluminiumproduction. The reason is, that most of the natural aluminiumcombinations contain too many impurities, especially iron and silicicacid, whereby the production of aluminium therefrom is rendered verydiflicult if not impossible. For the utilization of an aluminoussubstance for the electrolytic aluminium production the main conditionis, that it shall be as pure as possible.

This invention relates to the production of pure aluminium combinationssuitable for the aluminium production. It consists in that, asintermediate products or end products fluorine combinations of thealuminium are produced by converting an aluminium salt, not containingfluorine, by a metal fluoride in aqueous solution, suspension or insolid state at lower or higher temperature.

End products, capable of being direct-1y used for the aluminiumproduction and which are produced according to this invention, are,besides aluminium fluoride, specially the aluminium alkali-doublefluoride combinations, (cryolites). Utilizing aluminium fluoride, asintermediate product, even aluminium oxide may be obtained.

As aluminium salt not containing fluoride, any salt may be usedaccording to the invention, but aluminium nitrate or aluminium chlorideare to be used mainly.

According to whether aluminium fluoride or aluminium alkali-doublefluoride are to be produced, the reaction substances, aluminium salt notcontaining fluoride, and the metal fluorides, will be proportionedaccordingly. For the production of cryolites, of which the combination3NaFAlF possesses Serial No. 342,944, and in Austria March 5, 1928.

the greatest importance, aluminium nitrate will be reacted upon withsodium fluoride, or also potassium fluoride, ammonium fluoride and soforth, as illustrated for instance by the following equation:

The initial substances in the quantitative proportion stated are allowedto react in aqueous solutions or aqueous suspensions, preferably whileheated and agitated (stirred), whereupon after a certain time thecorresponding cryolites are produced in a pure form with formation ofthose metallic nitrates, the metallic fluorides of which are used forthe reaction.

As aluminium nitrate, and also aluminium chloride and aluminiumsulphate, contain Water of crystallization the components, after havingbeen previously intimately mixed, may also be allowed to act upon eachother by heating them within very Wide temperature limits, to producethe corresponding cryolites. It is possible to work with quite littleliquid and even with solid substances. The reaction of the aluminiumsalt not containing fluorine with the metallic fluoride can be carriedout under heating, stirring or other mechanical treatments, as well asunder pressure.

The aluminium salt not containing fluorine can be made from anyaluminous raw materials, as for instance clay, by treatment with strongmineral acid. Calcined clay is for instance extracted by means of amineral 30 acid at increased temperature. Besides the alumina theimpurities of the aluminous material, as iron, magnesium, alkali and thelike, go into solution, while eventually titanium and always the silicicacid remain insoluble. The dissolved alumina salt is filtered off withits impurities from the insolubles and the obtained solutions which withregard to aluminium may be basic or acid, are reacted upon withfluorides.

The mineral acid serving for treating the aluminous raw material can berecuperated, according to the invention, in the simplest manner from themetal salt free from fluorine resulting from the reaction of thealuminium salt with metallic fluoride, a portion or the total quantityof the metallic fluoride being eventually re-formed at the same time.The following equations illustrate a manner of carrying out therecuperation of the acid and simultaneous recuperation of the metalfluoride:

QKNO H SiF K SiF 2NNO K SiF (heat) =2KF SiF, CaJFQ H gsoi i For therecuperation of the acid I use, as shown by the above equations, acomplex hydrofluoric acid, which reacts with the resulting metal saltfree from fluorine. This acid, necessary for the treatment of thealuminous raw material, is liberated and separated from the complexhydrofluoric salt formed. I

The complex hydrofluoric salt is then decomposed by heating into simplemetallic fluoride and fluoride of an element supplying complex fluorinecombinations. The metallic fluoride is used for reaction with thealuminium salt, while the volatile fluoride (SW, of the equations) isused for re-fornr ing the complex hydrofluoric acid.

'The following equations show another possibility for the recuperationof the metallic fluoride required for the conversion of the aluminiumsalt into an aluminium fluorine combination CaIQ-i-ISiF NaNO3 Na SiF+Ca(NO Na SiF heat) =2NaF Sill From the sodium nitrate obtained isformed, by reaction with calcium fluoride amd silicon fluoride in presnce of acid, on the one hand, calcium nitrate and, on the other hand,sodium silico fluoride, the latter wilibe preferably used in suchquantity, 'th at a metal salt containing the acid bound in the aluminiumsalt and the base bound to the fluorine,and aluminium fluoride areformed, the latter being then submitted to a further treatment. In sucha reaction there is formed in the aqueous medium a precipitate,

' which contains aluminium fluoride and, when one has started fromferrous solutions, also a portion of the iron, while the main portion ofthe iron, and also the other soluble substances, remain in solution. Thesolution is then filtered off from the insoluble fluoride, and thealuminium fluoride is heated with such alkali-carbonates or alkalineearths, carbonates or caustic allra ics, or caustic all-:aline earths,that form with aluminium fluoride the corresponding fluorides andsoluble aluminates. The aluminate-solution is then filtered from theinsoluble fluoride, and the aluminates decomposed according to any ofthe commonly used methods.

These manners of carrying out the process may be illustrated by thefollowing examples slur, cKNo-i 2A1F3+6KOH= GKF 2mg OH) By this mannerof production an alumina is obtained which is practically perfectlypure, immaterial whether one starts from rich-or poor bauxites or fromaluminium sili cates of any kind, especially clay. An :al-uminate liquoris obtained, which owing to the decomposing of the raw material by anacid is absolutely free from silicic acid and owing to thesubsequenttreatment of the fluorides absolutely free from iron. The alumina teliquor is, notwithstanding impure initial substances, purer than thatwhich is obtained by any of the commonly used methods. The aluminiumhydroxide obtained contains at the utmost a few traces of sodium andfluorine, which are however not considered impurities in the fusedelectrolyte.

When for the extraction of the aluminous materials acids are used, whichpossess the property of not dissolving iron or of separating insolublesalts that take up the polluting iron, the further advantage is obtainedthat, in contradistinction I to the production of alumina-tes frombauxite, only very small quantities of iron have to be removed from thealuminate while preparing itfrom the aluminium fluoride and causticalkali or caustic alkaline earth.

Also in this form of carrying out" the new process the metallicfluorides used for the conversion with the aluminous salt tree fromfluorine, can always be rec'uperatcd and used again for the formation ofthe aluminii-un fluoride. The iron which pollutes the same can forinstance be extracted by dilute acids,

as the fluorides are as a rule very slightly soluble in acids.

Also the caustic alkalies or carbonates necessary as auxiliarysubstances may be used in the cycle. It may be specially mentioned that,in this form of carrying out the process, all residues are obtained in aform easy to filter.

The acid used for the decomposition of the aluminous raw material, andwhich according to the above described manner of carrying out theprocess is obtained in form of alkaline or earth alkaline salts, mayalso be recuperated, as mentioned above, with the aid of complexhydrofluoric acid and used for fresh treatment of the raw material.

Another possibility of producing alumina from the aluminium fluorideobtained in the reaction with metallic fluoride consists in acting underheating on the aluminium fluoride formed with saturated or superheatedsteam or other agents forming fluorine combinations at atmosphericpressure or in a gas diluted space or in a vacuum, with or withoutconducting over the aluminium fluoride gas or air. By this process theproduction of the aluminium oxide is much shortened, a number ofchemical and technical advantages being thus obtained. When thealuminium fluoride obtained is heated, for instance by steam,hydrofluoric acid and aluminium oxide are produced. The aluminium oxideobtained is extraordinarily pure; dense and heavy, and is obtained atonce in the calcined state so that generally no'subsequent calcinationis necessary. The measure presents the further considerable advantage,that at the same time an aluminium oxide is produced which is free fromalkali, and the thus obl tained product is further free from silicicacid, and in general free from all disturbing impurities which can notbe avoided in other processes of producing aluminium oxide. Thefollowing equations illustrate by way of exampl the manner of carryingout the process:

As the above equations show, the process can for instance be conductedso that the hydrofluoric acid is produced in the one phase of theprocess, which is then, caused to act upon the metallic salt obtained atthe production of the aluminium fluoride, whereat the mineral acidnecessary for the decomposition of the aluminous raw material, and atthe same time also the metallic fluoride serving for the conversion withthe aluminium salt free from fluorine to aluminium fluoride areobtained. This process can therefore be conducted in a cycle, and, onthe other hand,

combined with the production of valuable waste substances as forinstance potassium nitrate or hydrofluoric acid.

lVhen, however, it is desired to obtain, instead of hydrofluoric acid,silicon fluoride and pure alumina, pure silicic acid has to be added tothe aluminium fluoride and the thermic decomposition has to be carriedout in the presence of the same.

If the intention is, to obtain silicon fluoride as by-product, thissilicum fluoride can be used for the most various purposes in thechemistry of the fluorine combinations.

Another manner of carrying out the process, which also leads directly tothe aluminium oxide, produces the same with utilization of silicic acidand represents a complete cyclic process, will be explained by way ofexample by the following equations:

2. 4A1(NO +12NaF= min, meme,

5. 4NaNO 2H SiF 4HNO Qlla SiF 6. Quasar, W esir. tNaF 7. QCaF QSiF,aNaNO iHNO, znazsir 2Ca N03 2 HNO s. enasirfi +w esir. nea

9. QCaF -l- .ZSiFA diva-N0 iHNO zNazsir a (N03) 2 +Q1UHNOQ It isconsequently possible to carry out from the silicon fluoride obtained bytreatment of the same with hot steam a reaction in the sense of theEquation 11, i. e. with obtention of hydrofluoric acid. Thishydrofluoric m=heat.

acid is then made to act upon the salts of the I superheated steam in agas dilated space as, under these conditions, the decomposition can becarried out in the shortest possible time with quantitative yield andconsiderable sav ing of heat.

The process is carried through for example according to the followingexamples.

E sample I By decomposing clay with hydrochloric acid a solution isobtained containing in 100 kgs. 7.8 kgs. A1 0 and 0.68 kgs. F6 0 To thissolution of aluminiumchloride having a small excess of acid is added 9.6kgs. NaF, a quantity nearly equivalent to the A1 0 in solution. Underheating and stirring the reaction is carried through aluminium fluoridebeing precipitated, practically free from iron. This AlF is separatedfrom the solution and decomposed by heat while simultaneouslysuperheated steam and air is passed off the mass. A pure alumina andpure hydrofluoric acid are obtained.

'Ema-mple I I To a solution resulting from the decomposition of claywith sulphuric acid containing 7 .8 kgs. A1 0 and 0.38 kgs. Fe o assulphates, aqueous hydrofluoric acid (50%) is added the quantity ofwhich being nearly equivalent to the A1 0 in the solution. Aluminiumfluoride is thus precipitated, separated from the solution anddecomposed by heat in presence of steam. The hydrofluoric acid thusproduced is reused forpreparing the aluminium fluoride by introducingthe hydrofluoric acid into a solution of aluminium sulphate and on theother hand pure alumina is obtained. The sulphuric acid resulting at theproduction of aluminium fluoride may be used, even tually afterpurification from fluorine, for decomposing fresh quantitiesaluminiferous raw material and for producing aluminium sulphate.

Instead of alumina containing solutions obtained by treatment of aluminacontaining material with muriatic acid or sulphuric acid a solution ofaluminium nitrate may be used as starting material. For example, thestarting material may be 100 kg. of a solution obtained by treatingbauxite with nitric acid and containing 7.5 kgs. A1 0 and 0.09 kgs.F8303. p g

If silicon tetrafluoride is to be generated during the decomposing ofthe aluminium fluoride by heat, the obtained aluminium fluoride is driedby a little heat and mixed with silica, as pure precipitated silica.This mixture is heated up to 650 (1, under reduced pressure, while driedair is passed over the mass, whereby decomposition into alumina andsilicon fluoride takes place.

The silicium fluoride may be introduced into a solution of sodiumnitrate, obtainable during the process, in presence of a small amount ofacid as contact substance and calciumfluoride in order to generatesodium silicofluoride, which may be used according to the formulas namedabove.

The present invention permits further to produce in a very simple manneraluminous combinations which, owing to their purity, are specially wellsuited for the aluminiummetal production. It makes it possible toproduce pure alminium oxides as Well as aluminium alkali-doublefluorides, the agents required as auxiliary substances being conductedthrough the process in a cycle so that they can be used again and again.

I claim:

1. In the process of producing aluminium fluoride and double compoundsof aluminium, the steps which comprise producing a basic aluminiumnitrate solution by treating a fluorine free raw material containingalumina with a quantity of nitric acid insuificient to completely formAl(NO with the alumina present, and adding an alkali fluoride to theresulting basic aluminium nitrate solution. v

2. In the process as defined by claim 1, the further steps whichcomprise reacting the alkali nitrate, resulting from the production ofaluminium fluoride with hydrofluosilicic acid, to reconvert the alkalinitrate to nitric acid, for return to the cycle of operations, and analkali fluosilic-ic salt, decomposing the latter to obtain alkalifluoride for the process and silicon fluoride, and reconverting thesilicon fluoride into hydrofluosilicic acid for return to the cycle ofoperations.

3. In the process as defined by claim 1, the further steps whichcomprise reacting the resulting aluminium and fluorine containingcompounds with a caustic alkaline compound to form a soluble aluminatc,and converting the latter into aluminium hydroxide in a known manner.

4. In the process as defined by claim 1, the further steps whichcomprise reacting the resulting aluminium and fluorine containingcompounds with an alkali metal carbonate to form a soluble aluminate,and converting the latter into aluminium hydroxide in a known manner.

5. In the process as defined by claim 1, the further steps whichcomprise decomposing the aluminium and fluorine containing compounds byheat in the presence of available oxygen to form aluminium oxide and afluorine compound, and reacting the latter with an alkali compound so asto reform the alkali fluoride for the reaction with aluminium nitrate.

6. In the process as defined by claim 1,

the further steps which comprise decomposing the aluminium and fluorinecontaining compounds by steam to form aluminium oxide and hydrofluoricacid, and converting the latter into alkali fluoride for the reactionwith aluminium nitrate.

7. In the process as defined by claim 1, the further steps whichcomprise decomposing the aluminium and fluorine containing compounds inthe presence of available oxygen While maintaining the reaction chamberbelow atmospheric pressure to form aluminium oxide and a fluoride forsubsequent reaction with an aluminium salt.

8. In the process as defined by claim 1, the further step whichcomprises decomposing the aluminium and fluorine containing compounds bysteam in the presence of silicic acid to form aluminium oxide and afluoride for subsequent reaction with an aluminium salt.

In. testimony that I claim the foregoing as my invention. I have signedby name.

MAX BUCHNER.

